This invention pertains to a power control circuit, and more particularly to a circuit for controlling power delivery to different AC loads by trigger-pulse-controlling a switching circuit.
When AC loads are reactive, with inductance predominant in the reactance, current lags voltage by an amount which is related to the proportion of inductance to resistance in the load. This situation occurs for many common types of AC loads, such as fluorescent lamp ballasts, motors and transformers. When these types of loads are used with solid-state dimming components, such as silicon-controlled rectifiers (SCR's), triacs or transistors, it becomes necessary to allow for the current lag relative to voltage.
SCR's, which are commonly used to control the supply of AC power to a load, allow the conduction of a positive current after they have been turned on by a suitable trigger pulse. Conduction will be initiated if a positive current flow exists when the trigger pulse is applied. Once in a conducting state, an SCR will continue to conduct until the current flow reaches a minimum positive value determined by the SCR characteristics.
The conduction angle is that portion of a half cycle of voltage during which it is desired to cause an SCR to conduct. Thus, a conduction angle of 120.degree. would indicate that conduction should begin 60.degree. after initiation of the half cycle and continue to its end. For larger conduction angles, during which conduction is desired over most of the half cycle, if a short duration triggering pulse is applied, it may occur while the voltage is positive but the current is negative, thereby preventing conduction of the SCR. In order to overcome this situation, circuits conventionally provide a triggering pulse equal to the conduction angle. Thus, the triggering pulse is maintained even after the current changes from negative to positive.
Due to the current lag, the SCR conducts for a period of time equal to the conduction angle plus the lag angle minus any initial portion of the conduction angle during which current is negative. As an example, assume that a circuit has a lag angle of 60.degree. and a conduction angle of 150.degree.. Since the current becomes positive 60.degree. after the voltage, it is negative for the first 30.degree. of the conduction angle (150.degree.-(180.degree.-60.degree.)=30.degree.). The SCR will conduct for 150.degree. plus 60.degree. less 30.degree. or 180.degree.--the full half cycle of positive current flow.
It can therefore be seen that a conventional power control circuit of the type herein described provides a triggering pulse equal to the conduction angle when in fact it is only necessary to apply a pulse sufficient in length to assure existence of the pulse when the current becomes positive.
It is therefore a general object of this invention to provide a trigger-pulse-controlled circuit which uses less energy in controlling the switching circuit than conventional circuits.
More specifically, it is an object of this invention to provide a circuit controlled by a triggering pulse having a maximum width which is no less than a known maximum lag angle for the circuit.
The preferred embodiment of this invention includes a switching circuit which utilizes silicon-controlled rectifiers for controlling power delivery, and a trigger-pulse generator which generates a pulse having a maximum width which is no less than a known maximum phase lag for the types of loads which may be connected to the circuit.
In such a circuit, it can be seen that for large conduction angles, the triggering pulse will always exist when the current switches from a negative to a positive direction. Additionally, it only uses enough energy to produce a pulse which will cover essentially all lag angle conditions.
These and additional objects and advantages of the present invention will be more clearly understood from a consideration of the drawings and the detailed description of the preferred embodiment.